Button and color enhancements (#24989) (#25176)

Backport #24989. Clean cherry-pick aside from one small conflict with
divider.

- Various corrections to button styles, especially secondary
- Remove focus highlight, it's annoying when it stays on button after
press
- Clearly define ghost and link buttons with demos in devtest
- Remove black, grey and tertiary buttons, they should not be used
- Make `arc-green` slightly darker

<img width="1226" alt="image"

src="8d89786a-01ab-40f8-ae5a-e17f40e35084">
<img width="1249" alt="image"

src="83651e6d-3c27-46ff-b8bd-ff344d70e949">

templates/repo/diff/section_unified.tmpl

@@ -41,7 +41,7 @@
 			{{$inlineDiff := $section.GetComputedInlineDiffFor $line $.root.locale -}}
 			<td class="lines-escape">
 				{{- if $inlineDiff.EscapeStatus.Escaped -}}
-					<button class="toggle-escape-button button button-ghost" title="{{template "repo/diff/escape_title" dict "diff" $inlineDiff "locale" $.root.locale}}"></button>
+					<button class="toggle-escape-button btn interact-bg" title="{{template "repo/diff/escape_title" dict "diff" $inlineDiff "locale" $.root.locale}}"></button>
 				{{- end -}}
 			</td>
 			<td class="lines-type-marker"><span class="gt-mono" data-type-marker="{{$line.GetLineTypeMarker}}"></span></td>